Physiological Role of Ca2+-Permeable Nonselective Cation Channel in Endothelin-1-Induced Contraction of Rabbit Aorta

We previously showed a role for a nonselective cation channel (NSCC) in the ETA-dependent action of endothelin-1 in mouse fibroblast and rabbit aortic smooth-muscle cell. To clarify the physiological significance of NSCCs in endothelin-1 (ET-1)-induced vasocontraction, we examined the effects of NSCC blockers such as mefenamic acid and SK&F 96365 on the contractions of deendothelialized rabbit aortic rings induced by a low (10[-10] M) or high (10[-8] M) concentration of ET-1. Mefenamic acid (< or =10[-3] M) had little effect on the contraction induced by 45 x 10(-3) M K+ or 1 x 10(-6) M Bay K-8644 in combination with 15 x 10(-3) M K+, indicating that it does not affect voltage-operated calcium channels (VOCs) and contractile mechanisms. The contraction by a low concentration of ET-1 was abolished after removal of extracellular Ca2+, but it was reduced only to 50% by a maximally effective concentration (10[-5] M) of nifedipine, an inhibitor of L-type VOCs (L-VOC). Mefenamic acid and SK&F 96365 inhibited the ET-1-induced contraction with 50% inhibitory concentration (IC50) values of 10(-4) M and 2 x 10(-5) M, respectively, and abolished it at 10(-3) M and 10(-4) M. By contrast, nifedipine, mefenamic acid, or SK&F 96365 had little effect on the contraction by a high concentration of ET-1. The contraction induced by a low or high concentration of ET-1 was abolished by an ETA antagonist, BQ-123, but not by an ETB antagonist, BQ-788. These results demonstrate that the contraction induced by ET-1 is totally mediated exclusively by ETA, but that Ca2+ entry through NSCCs in addition to L-VOCs plays an important role in contractions induced by low concentrations of ET-1, whereas it plays only a minor role in contractions induced by high concentrations of ET-1.